This invention generally relates to welding or heating systems. In particular, the invention relates to plasma arc cutting units.
Plasma arc cutting is a process in which an electric arc is used to cut a metallic workpiece. Generally, plasma arc cutting uses an electric arc between an electrode and the metal to be cut. The arc creates a plasma that cuts the metallic workpiece. Plasma cutters are often used in applications such as building maintenance (structural or HVAC) where they are carried from job to job, and thus are preferably highly portable.
A typical prior art plasma arc cutter includes a. power supply, an air supply and a plasma torch. A plasma torch is used to create and maintain the arc and the plasma jet that perform the cutting. A plasma cutting power supply receives an input voltage (from a power line or generator) and provides an output voltage to a pair of output terminals, one of which is the electrode and the other of which is connected to the workpiece. There are numerous types of known plasma arc cutting power supplies, such as magnetic power supplies, inverter power supplies, phase control power supplies, and choppers or secondary switchers. A typical plasma cutting power supply includes one or more of each of the following: input power lines, transformers, converters/inverters, output power lines, and controllers, which cooperate to provide power to the electrode for maintaining the arc.
An air supply is used with most plasma cutters to help start the arc, control the torch, and cool the torch. For example, U.S. Pat. No. 4,791,268 to Sanders et al. describes a plasma torch controlled with air. A movable electrode is the cathode and a fixed nozzle is the anode. A pilot current is provided to the cathode and anode, which are forced into contact by a spring. After electrical current begins to flow from the electrode to the nozzle, gas or air supplied to the torch counteracts the spring force and moves the electrode away from the nozzle. This breaks the electrical contact between the electrode and the nozzle and creates the pilot arc. Also, as the electrode moves away from the nozzle, it opens a nozzle orifice (connected to the air supply), and a plasma jet is created.
The plasma jet causes the arc to transfer (at least in part) to the work piece, thus initiating cutting. Electronics in the power source sense when the arc has transferred and then supply a main cutting current of greater amperage after the transfer has occurred. Also, the torch tip is disconnected (electrically), interrupting the pilot current path. Thus, the current is used to cut the workpiece, and follows a path including the positive terminal, the workpiece and the electrode.
Plasma arc torches are widely used in the cutting or marking of metallic materials. A plasma torch generally includes an electrode and a nozzle having a central exit orifice mounted within a torch body, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
One known configuration of a plasma arc torch includes one or more cables connecting the torch to the power supply to provide the torch with electrical current and gas. The connection of the cable(s) to the power supply must be rugged to handle the strain placed on the cable as it is manipulated in order to place the plasma arc torch in a suitable position to cut or mark a workpiece.
The cable(s) used to connect the torch to the power supply can be a single integral cable having a gas hose located in the middle of the cable and electrical conductors and fillers arranged symmetrically around the gas hose. A jacket material is extruded over the gas hose, electrical conductors and fillers. A strain relief mechanism can be attached to the jacket to handle loads applied to the cable. The jacket, gas hose, electrical conductors and fillers are anchored together over a barb-type fitting. A clamp, which acts as the strain relief mechanism, is applied to grab and hold the jacket to prevent relative axial motion (or twisting) of the cable components. The stress through the cable is absorbed by the clamp and transferred to the chassis of the power supply through a mechanical connection.
Cables with a clamp designed to prevent axial motion or twisting of cable components are disclosed in the prior art. The cable used in one plasma arc torch system is connected to the power supply by a threaded quick disconnect connector. A quick disconnect connector is advantageous in that it simplifies torch removal but is expensive to make. In another example of an integral cable with a clamp for preventing axial motion (or twisting) of cable components, stress is absorbed by the chassis of the power supply through a tool-tightened nut. A tool-tightened nut is advantageous in that it is inexpensive.
U.S. Pat. No. 6,4120,631, assigned to Hypertherm, Inc., discloses a plasma arc torch for piercing or cutting a workpiece. The plasma arc torch system includes a torch body, a power supply and a cable with two ends. One end of the cable is connected to the torch body, while the other end of the cable is attached to a strain relief system to couple the cable to the power supply. The strain relief system includes a positive axial restraint component for restraining axial movement of the cable relative to the power supply and a positive rotational restraint component for restraining rotational movement of the cable relative to the power supply. The positive axial and rotational restraint components are independent components arranged in a spaced relationship to each other. In one embodiment, the positive axial restraint component comprises a quick disconnect pneumatic connection attached to the cable and a quick disconnect pneumatic connection receptacle (which receives the pneumatic connection and itself is coupled to a supply of gas or air) positioned inside the power supply housing. The positive rotational restraint component comprises a shaped boot attached to the torch cable and a mating receptacle formed in the power supply housing. The shaped boot and the mating receptacle are designed to prevent rotation of the torch cable when the shaped boot is inserted in the mating receptacle. The electrical connection between the torch cable and the supply unit is independent of the quick disconnect pneumatic connection. Both the electrical connection and the quick disconnect pneumatic connection are located inside the housing.
There is a need for a torch cable in which both the electrical connection and the pneumatic connection can be made concurrently in one motion by coupling the end of the torch cable to the exterior of the supply unit. As used herein, the term “torch cable” includes both a cable having one end plugged directly into a supply unit and having a torch connected at the other end as well as a cable having one end connected to the supply unit via one or more extension cables and having a torch connected to its other end. In the latter case, the cable carrying the torch and the one or more extension cables, when connected in series, form a “torch cable”.